As is well known in the field of hydraulic engineering, there is an ongoing need to inhibit erosion caused by rivers, streams and other waterways both natural and man made which occurs at locations where there is a change in grade. Well known in the prior art are various types of hydraulic energy dissipation devices which are commonly referred to under the collective term "energy dissipators", and are used to provide erosion control protection by serving as, among other things, dam spillways, drop structures in natural streams or man made channels, and grade control structures in natural streams or man made channels. The significant resources devoted by many governmental agencies to protect civil structures such as canals, dams or other waterways constructed of earthen materials from erosion has resulted in the development of a relatively wide range of prior art energy dissipators and other erosion protection systems.
One category of prior art energy dissipator is adapted to develop a high velocity at the toe or bottom of the drop, with dissipation of the hydraulic or kinetic energy being accomplished by a hydraulic jump. These types of energy dissipators typically shorten the length of the hydraulic jump and subsequently reduce the distance of high velocities downstream of the toe which would otherwise cause scour. In the prior art, there are numerous designs of these types of energy dissipators currently in use, with perhaps the most common being a stilling basin which incorporates special appurtenances (i.e., chute block, end sills, baffles, etc.) which tend to stabilize the hydraulic jump and improve its performance.
Other types of prior art energy dissipators currently under research include stepped spillways, roller compacted concrete (RCC), gabions, riprap, baffle apron drops, geotextiles, and concrete block revetment systems. However, as will be discussed below, these prior art energy dissipators each possess certain deficiencies which detract from there overall utility.
Gabions are wire baskets which are filled with rock and anchored to slopes for erosion protection. Though gabions have been successfully used for building dams with gabion spillway weirs, research has indicated that though they may perform well if anchored properly, they undergo considerable deformation under certain flow conditions. More particularly, it has been determined that structural deformation of gabions could occur for flows in excess of sixteen (16) CFS or velocities between fifteen (15) and seventeen (17) feet per second. For flows in excess of these parameters, additional strengthening is typically required for the gabions.
With regard to riprap, research is currently underway regarding the use of riprap as a means of reducing toe velocities using rock chutes. Thus far, this research has indicated that there are limiting factors associated with the structural stability of riprap on steep slopes subject to high flows which severely limits its utility. In particular, modeling has demonstrated that riprap scaled to represent a median twenty-four (24) inch diameter rock on a 3 to 1 slope was only able to withstand a scaled unit discharge of under twenty three (23) CFS per foot. Flows in excess of this value exhibited failure by materials being dislodged and transported down the slope (chute). At present, the difficulties associated with accurately predicting the behavior of riprap protection has mitigated against its recommended usage as protection from overtopping flows of any significant magnitude.
Though roller compacted concrete (RCC) has proven to be very effective in protecting against erosion, the protection imparted thereby is attributable to the thickness of the concrete overlay alone. Though the applications for roller compacted concrete are widespread, they rely on the strength of the material and the cover thickness to provide erosion protection. It has been determined that subjecting the materials to high velocity flow would likely degrade the protective system. Additionally, the installation techniques associated with roller compacted concrete are generally economical only for the placement of large qualities of material, and further require easy site access. Moreover, roller compacted concrete may significantly impact the surrounding environment.
Though baffle apron drops have been used successfully in canal design, the systems have not had extensive use in flood control applications and are susceptible to damage from debris. Additionally, testing of geotextiles has indicated that failure occurs at relatively high velocities, with such failure believed to be caused by poor anchoring or stretching of the material.
Concrete block revetment systems (articulating blocks) are generally cable-tied together and anchored to the embankment, with grass being used to cover over the voids. However, the use of concrete block revetment systems is largely limited to erosion protection, with most of these systems being designed to prevent river bank erosion. Thus far, two such systems have been tested and are in limited use for overtopping protection, but are considered to be unsuitable for high flow velocities due to the energy dissipation properties being minimal.
Step spillways have been in use for thousands of years, and are currently experiencing a re-emergence. In this respect, the step spillway is currently under strong research, with many hydraulic researchers believing that step spillways will be included with the more classical types of energy dissipators currently being used in erosion protection applications. The stepped spillway is a simple form of a rough channel wherein a stable rolling vortex is developed within each step. This rough channel does not allow the velocity down the drop to reach the velocity that would occur on a smooth spillway, with these reduced toe velocities having an effect on the stilling basin design at the toe. The stable rolling vortex created within each step of the stepped spillway as discharge flows down the drop dissipates a considerable amount of hydraulic or kinetic energy. However, although dissipating energy, the vortex also acts as a "cushion" for skimming flows. This particular hydraulic characteristic results in little lateral movement of the water or discharge as it flows down the drop and velocities that are basically two dimensional.
The shortcomings of the above-described prior art energy dissipators are overcome by the offset stepped spillway constructed in accordance with the present invention which dissipates energy at rates exceeding several magnitudes above any known prior art energy dissipation system. In this respect, the offset stepped spillway of the present invention has the ability to dissipate large amounts of kinetic energy on a continuous basis, and possesses several hydraulic characteristics which represent a significant departure from those associated with prior art stepped spillways. In particular, the offset steps and stacking pattern in the present spillway annihilates any semblance of a stable vortex at each step, and creates high gradient velocity zones. Additionally, the offset steps and stacking pattern creates a high lateral diffusing of velocities, and thus transforms two dimensional flow into three dimensional flow. Moreover, the offset steps and stacking pattern is believed to generate slight vortex rollers, with the offset steps creating a shear zone where there is a negative (upslope) velocity component coming in contact with the primary positive (downslope) velocity component. This negative-positive contact is believed to occur on the drop and interferes with the primary direction of flow which reduces the velocity in the primary direction and thus dissipates additional kinetic energy.